In wireless communications systems, a mobile station (MS) maintains communication with a serving base station (BS) in normal mode operation, during which the MS actively receives and transmits data packets. In order to minimize power consumption, the MS sometimes enters sleep mode operation, during which the MS conducts pre-negotiated periods of absent time from the serving BS air interface. Thus, in addition to minimize power consumption, sleep mode operation is also designed to reduce usage of the serving BS air interface resource, and to mimic certain desirable traffic characteristics. When sleep mode operation is active, a series of alternating listening windows followed by sleep windows are provided for the MS. In each listening window, the MS is expected to receive and transmit data packets as in normal mode operation. In each sleep window, the serving BS shall not transmit any data packets to the MS.
FIG. 1 (Prior Art) illustrates examples of sleep mode operation in an IEEE 802.16e wireless communications system. As illustrated in FIG. 1, three types of power saving classes (PSCs) are defined based on different traffic characteristics. For non-real-time variable rate (NRT-VR) traffic and best effort (BE) traffic, a PSC of type I is defined. In type I PSC, the MS is provided with a fixed listening window length for monitoring incoming protocol data units (PDUs). The MS starts with an Initial Sleep Window Length TMIN (S0=TMIN), and each subsequent sleep window Sk grows exponentially until reaching a Final Sleep Window Length TMAX (Sk=min {TMIN*2k, TMAX}). When a traffic indication message indicates positive traffic or when there are incoming PDUs in a subsequent listening window, the MS goes back to normal mode operation. For real-time variable rate (RT-VR) traffic and unsolicited grant service (UGS) traffic, a PSC of type II is defined. In type II PSC, each sleep window has a fixed length (Sk=S0). For multicast or management traffic, a PSC of type III is defined. In type III PSC, the MS enters one fixed-length sleep window (S0=final sleep window) to process the multicast or management traffic and then goes back to normal mode operation.
FIG. 2 (Prior Art) illustrates a problem of power waste under PSC of type I for NRT-VR/BE traffic. In the example of FIG. 2, variable-sized data bursts are generated with variable arrival intervals from the serving BS. The MS goes back to normal mode operation when the traffic indication message indicates positive traffic. The length of sleep window is reset to the Initial Sleep Window Length when sleep mode operation is reactivated. For NRT-VR/BE services, there generally would be long silence time between two successive data bursts. However, because the length of the sleep window is reset to the Initial Sleep Window Length, the MS unnecessarily wakes up during the long silence and extra power is wasted.
FIG. 3 (Prior Art) illustrates a problem of power waste under PSC of type II for RT-VR traffic. In the example of FIG. 3, each listening window has a fixed length. For RT-VR traffic, variable-sized data bursts are generated periodically with fixed arrival interval from the serving BS. Because each listening window has a fixed length, the MS keeps monitoring incoming PDUs even when no data packets are being received. Thus, the fixed listening window length approach is inefficient and extra power is wasted.
FIG. 4 (Prior Art) illustrates a problem of unnecessary latency under modified version of PSC of type II for RT-VR traffic. In the example of FIG. 4, each listening window has an adjustable length, but each sleep window has a fixed length. For RT-VR traffic, variable-sized data bursts are generated periodically with fixed arrival interval from the serving BS. If the incoming PDUs arrive at a time that the MS is sleeping, then the MS has to wait for the next listening window to receive the PDUs. Thus, because each sleep window has a fixed length, unnecessary latency is introduced.
Various solutions have been sought to solve the problems of the above-described sleep window-based sleep mode operation. In U.S. Pat. No. 7,289,804, the length of each initial sleep window is not fixed, but instead changes based on a counter for each sleep mode. In US patent publication 2008/0009328, the length of each sleep window grows exponentially, with additional parameters such as a sleep mode ratio to modify the speed of the growth. In US patent publication 2008/0075026, the length of a listening window can be extended according to a data reception timer. In LTE systems (e.g. 3GPP TS 36.321), a fixed DRX cycle is applied with adjustable On-duration time. These solutions have not eliminated the above-described energy waste and latency problems. In addition, the current PSC mechanism is unsuitable for the application for multi-rate traffic transmission. A solution is sought.